Switching Mode Power Supplies (SMPS) are used in a variety of portable electronic devices including laptop computers, cellular phones, personal digital assistants, video games, video cameras, etc. They may convert a dc signal at one voltage level to a dc signal at a different voltage level (this is a dc-dc converter), an Alternating Current (ac) signal to a dc signal (this is an ac-dc converter), a dc signal to an ac signal (this is a dc-ac converter), or an ac signal to an ac signal (this is an ac-ac converter). In many applications, power converters have standby power guidelines that specify the amount of power they can consume. As these guidelines become more stringent, power converter manufacturers are faced with improving the active mode efficiency during light load conditions and during no-load conditions. For example, power consumption guidelines for power converters are fast approaching the specification of consuming less than 100 milliwatts when left connected to the mains in a no-load condition.
Today's high efficiency SMPSs use synchronous rectification to achieve the desired efficiency in their power stage. A controller using synchronous rectification controls a MOSFET switch that bypasses a standard rectifier for most of its conduction time. The MOSFET is typically referred to as an SR MOSFET switch. The SR MOSFET switch is used as a bypass element because it has a lower voltage drop compared to a standard diode or a Schottky rectifier. This lower voltage drop decreases the power loss and increases the efficiency of the SMPS power stage. In a zero current detection method the voltage drop between the drain and the source of the SR MOSFET is used to determine when to turn on or turn off the SR MOSFET. In this technique, the turn-on and turn-off threshold of the secondary current is usually equal to or near zero. Because the turn-off voltage between the drain and the source of an SR MOSFET is equal to or near zero, offsets in a current sense comparator can cause a significant turn-off current error. Another drawback is that the propagation delay of the current sense comparator should be as low as possible to turn off the SR MOSFET in a timely fashion, i.e., as soon as possible after detection of a zero current condition.
One technique for detecting a zero current condition involves the use of two comparators, where one detects the turn-on threshold voltage and the other detects the turn-off threshold voltage. Drawbacks with this technique include the need for isolated pnp bipolar transistors in the differential input stage to have a small input offset voltage and a low propagation delay, imprecision, and the need for an additional input/output pin for setting the turn-off threshold voltage.
Accordingly, it would be advantageous to have a circuit and a method for detecting a zero current condition, providing offset nullification, and capable of setting the turn-off threshold voltage without increasing the number of input/output pins. It would be of further advantage for the circuit and method to be cost efficient to implement.